1. Field of the Invention
The present invention relates generally to polymeric compositions for use as absorbent materials for aqueous fluids and to a process for the preparation of these compositions; and more specifically to improved superabsorbent polymeric compositions prepared from monomer including water soluble acrylate salt.
2. Brief Summary of the Prior Art
"Superabsorbents" are water insoluble materials which are capable of absorbing and retaining large amounts of water or other aqueous fluids in comparison to their own weight. The term "superabsorbent" is commonly used in the industry to refer to materials capable of absorbing at least fifteen times their own weight. Superabsorbents are used in a variety of disposable diaper and catamenial products. Disposable goods manufactured using superabsorbents can be more comfortable, less bulky, and longer lasting than similar products made with traditional absorbents such as cellulose fibers. Disposable diapers and catamenial products containing superabsorbents are disclosed, for example, in U.S. Pat. Nos. 4,676,784, 4,673,402, 4,670,011 and 4,610,678. Superabsorbents can be supplied in a variety of physical forms including free-flowing powders. Superabsorbents are typically hydrogel-forming polymer compositions: hydrophilic polymeric compositions which are crosslinked to insolublize them. Synthetic polymeric compositions prepared from monomer which includes a carboxylate functional group, such as sodium acrylate, are commercially important superabsorbents. Synthetic polymeric superabsorbents are well known in the art, and are disclosed for example, in U.S. Pat. No. 4,286,082 (acrylic acid, alkali metal acrylate, and a crosslinkable comonomer polymerized in presence of surface active agent); U.S. Pat. No. 4,654,039 (low temperature, low concentration polymerization of acid monomers to form high strength hydrogel); U.S. Pat. No. 4,167,464 (photopolymerized copolymers of acrylic acid) (C.sub.10 -C.sub.30)alkyl(meth)acrylates, and (C.sub.1 -C.sub.9)alkylacrylates); U.S. Pat. No. 4,354,487 (graft-polymerized polyacrylate-cellulose fiber composites); U.S. Pat. No. 4,558,100 (homopolymer of saponified, crosslinked (meth)acrylonitrile); U.S. Pat. No. 4,587,319 (copolymer of acrylic acid and allyl oligosaccharide); and U.S. Pat. No. 4,613,543 (polyacrylate/polyurethane interpenetrating polymer network). Superabsorbents can also be prepared from natural polymeric materials, such as disclosed in U.S. Pat. No. 4,483,950 (dextrin-extended starch-polyacrylonitrile graft copolymer); U.S. Pat. No. 4,650,716 (non-fibrous carboxymethyl cellulose); and U.S. Pat. No. 3,935,099 (starch-polyacrylonitrile). A summary of U.S. patents relating to superabsorbent polymers is given in U.S. Pat. No. 4,551,191 (Column 6). Japanese Unexamined Patent Application No. 56-161412 discloses a water-absorbent resin polymerized from monomer including (meth)acrylic acid and a copolymerizable sulfonic acid.
Ideally, superabsorbents are completely insoluble materials which are highly swellable by urine and other aqueous body fluids. In preparing synthetic polymeric superabsorbents a compromise must be drawn between insolubility and swellability: as the extent to which the polymer is cross-linked increases, the proportion of the polymeric composition which is soluble (soluble fraction) decreases, but so does the swellability and concomitant absorbent capacity of the polymeric composition. Consequently, synthetic polymeric superabsorbents are typically only lightly crosslinked to render them insoluble and have relatively high soluble fractions.
A number of approaches have been suggested for reducing the soluble fraction while only lightly crosslinking the polymer to retain high absorbence capacity. For example, U.S. Pat. No. 4,473,689 discloses minimization of initiator concentration by gradual addition of initiator to the polymerization medium. U.S. Pat. No. 4,654,039 favors polymerizing acidic monomers in their free acid, non-neutralized forms at relatively low polymerization temperatures and at relatively low monomer concentrations.
The polymers can be crosslinked by a variety of means. For example, crosslinking can occur during polymerization through incorporation of copolymerizable vinyl multifunctional comonomer, or subsequent to polymerization by reaction between carboxyl groups pendent from individual polymer molecules and a suitable multifunctional crosslinking agent. According to U.S. Pat. No. 4,286,082 superabsorbents can be prepared by polymerizing acrylic acid which has been at least 50% neutralized to a soluble acrylate salt form.
A polymeric suberabsorbent's capacity to swell is a function of both the average molecular weight of the polymer (assuming an uncrosslinked composition) and the crosslink density. High average molecular weight is correlated with the ability to retain absorbed aqueous fluids against applied pressure. Superabsorbents are generally characterized by the amount of fluid absorbed in a specified time ("capacity") and by the capacity retained once they are swollen and placed under an applied pressure ("retention"). A high average molecular weight is achieved through optimization of polymerization conditions. While ideally each polymer chain should be connected by the minimum number of crosslinks which insures insolubility, in practice a distribution in the number of crosslinks per chain is realized, and maximum swellability occurs while there is yet a significant proportion of uncrosslinked chains. When this soluble fraction is too high, a substantial portion of the polymer composition will dissolve to form a polymer solution and not participate in the desired absorption. The polymer solution will have a high viscosity relative to water or body fluids, and its presence may retard wicking by absorbent structures containing the superabsorbent. The dissolved polymer may even diffuse to the skin where it may cause irritation. Conversely, when a soluble fraction is too low, many chains will have been excessively crosslinked and the swellibility and corresponding absorption capacity will be restricted.
When superabsorbents are polyelectrolytes, such as carboxylate-functional superabsorbents, their absorption capacities depend on the ionic strength of the absorbed aqueous fluids. The absorption of body fluids such as urine and blood, which contain on the order of 1% by weight of various dissolved salts, is substantially less than the absorption of electrolyte-free water.
Polyelectrolyte-type superabsorbents are the most efficient of commerically available materials. They are typically prepared by at least partially neutralizing an aqueous solution of acrylic acid with an alkali metal base, such as sodium hydroxide, and subsequently polymerizing the acrylate monomer. However, the extent to which the acrylic acid is neutralized prior to polymerization represents an undesirable compromise.
In the first place, process considerations suggest that complete neutralization of the acid monomer is undesirable in some processes because phase separation is likely to occur during the polymerization under these conditions. Phase separation may result, depending on the specific process used, in inconsistent production runs, failure to meet specifications, and a host of associated problems. Thus a prudent process would seem to include only partial neutralization of the acid monomer.
On the other hand, the polymerization product is typically dried to a solid and granulated. Residual acrylic acid in the polymer tends to volatilize when the hydrogel is heated. Further, polymerized acrylic acid tends to depolymerize at the elevated temperatures used in drying the product. Because acrylic acid has some toxicity it is desirable to employ additional expensive equipment in the process to control, collect and dispose of the volatilized monomer.
Further, neutralizing the carboxylic acid residues in the polymeric compositions is not always practical, because the product is a hydrogel. Post polymerization neutralization requires additional capital equipment and extends processing time, raising the cost of the superabsorbent product. These considerations counsel that the acid monomer be substantially completely neutralized prior to polymerization.
Given the substantial commercial importance of polyacrylate-type superabsorbents, there is an unmet need for process for producing polyacrylate-type superabsorbents which avoids the pitfalls and compromises of prior art processes, and permits the economical manufacture of high quality superabsorbents, with minimal phase separation, and without requiring additional equipment for collecting and retaining noxious gaseous byproducts of the drying process, or for post-polymerization neutralization.